Function here describes y(x), x is the input, y is the output.
A single input or x-value can only yield 1 output or y-value otherwise it is NOT a function.
For example if x = 4 but y could be 2 or -2, Then this is not a y(x) function.
THis can happen when y has an exponent.
x = y^2 ----> 4 = 2^2 = (-2)^2
Therefore answer is b) x = y^2-4y+2
